CB-201: cover losing completion CAS

fleetd/src/test/java/dev/ltms/fleet/session/SessionManagerTest.java

@@ -349,6 +349,51 @@ class SessionManagerTest {
                 "onDelivered must refuse a backend-error member");
     }
 
+    @Test
+    void losingCompletionDoesNotReleaseOrClearABackendErrorMember() {
+        assertLosingCompletionLeavesBackendErrorIntact(1, false,
+                "a stale completion must not release a backend-error member at the context cap");
+        assertLosingCompletionLeavesBackendErrorIntact(0, true,
+                "a stale completion must not clear the context of a backend-error member");
+    }
+
+    private void assertLosingCompletionLeavesBackendErrorIntact(int contextCap, boolean clearAfterTurn,
+                                                                  String protectedAction) {
+        FakeHerdr herdr = new FakeHerdr();
+        FleetConfig.Profile cfg = new FleetConfig.Profile(
+                "ltms-local", "http://gx00.gw:8000", "coder", null, "FLEETD_WORKER_TOKEN",
+                List.of("ccs", "ltms-local"), "tab", "fleetd-workers",
+                "worker: {profile} #{n}", null, null, null);
+        ClaudeCodeLauncher delegate = new ClaudeCodeLauncher(new AgentControl(herdr), new WorkspaceControl(herdr),
+                new SubscriptionGuard(Set.of("gx00.gw")), Map.of(cfg.profile(), cfg), cfg.profile(), _ -> null);
+        ClearContextSpyLauncher launcher = new ClearContextSpyLauncher(delegate);
+        java.util.concurrent.atomic.AtomicReference<SessionManager> manager = new java.util.concurrent.atomic.AtomicReference<>();
+        java.util.concurrent.atomic.AtomicReference<String> terminal = new java.util.concurrent.atomic.AtomicReference<>();
+        java.util.concurrent.atomic.AtomicBoolean armed = new java.util.concurrent.atomic.AtomicBoolean();
+        SessionManager sessions = new SessionManager(launcher, new RecordingWorktrees(), () -> {
+            if (armed.compareAndSet(true, false)) {
+                manager.get().onBackendError(terminal.get(), "backend exited during completion");
+            }
+            return 1;
+        }, contextCap, clearAfterTurn);
+        manager.set(sessions);
+        MemberSession session = sessions.acquire("ltms-local", null, "/caller", "term_primary");
+        terminal.set(session.terminalId());
+        sessions.asPresence().markPresent(session.terminalId());
+        sessions.onDelivered(session.terminalId(), TestTurnTokens.inert(session.terminalId()));
+        armed.set(true);
+
+        assertFalse(sessions.onTurnCompleteWithPostAction(session.terminalId()),
+                "the completion CAS loses after the injected backend error");
+
+        assertTrue(sessions.get(session.paneId()).isPresent(), protectedAction);
+        MemberSession updated = sessions.get(session.paneId()).orElseThrow();
+        assertEquals(MemberSession.State.BACKEND_ERROR, updated.state());
+        assertEquals("backend exited during completion", updated.failureReason());
+        assertFalse(herdr.called("pane.close"), protectedAction);
+        assertEquals(0, launcher.clearContextCalls(), protectedAction);
+    }
+
     @Test
     void backendErrorForUnknownTargetIsWarnedAndDoesNotCreateASession() {
         LoggerContext ctx = (LoggerContext) LoggerFactory.getILoggerFactory();
@@ -1104,6 +1149,76 @@ class SessionManagerTest {
         }
     }
 
+    /** Delegates all launcher work while recording context clears. */
+    private static final class ClearContextSpyLauncher implements PeerLauncher {
+        private final PeerLauncher delegate;
+        private int clearContextCalls;
+
+        ClearContextSpyLauncher(PeerLauncher delegate) {
+            this.delegate = delegate;
+        }
+
+        @Override
+        public Set<Capability> capabilities() {
+            return delegate.capabilities();
+        }
+
+        @Override
+        public Set<Capability> capabilitiesFor(String profileName) {
+            return delegate.capabilitiesFor(profileName);
+        }
+
+        @Override
+        public PeerHandle spawn(SpawnRequest req) {
+            return delegate.spawn(req);
+        }
+
+        @Override
+        public Set<String> profiles() {
+            return delegate.profiles();
+        }
+
+        @Override
+        public String defaultProfile() {
+            return delegate.defaultProfile();
+        }
+
+        @Override
+        public String effectiveCwd(SpawnRequest req) {
+            return delegate.effectiveCwd(req);
+        }
+
+        @Override
+        public List<String> parityOverlay(String profileName) {
+            return delegate.parityOverlay(profileName);
+        }
+
+        @Override
+        public List<?> list() {
+            return delegate.list();
+        }
+
+        @Override
+        public int reapOrphanWorkers() {
+            return delegate.reapOrphanWorkers();
+        }
+
+        @Override
+        public void stop(String id) {
+            delegate.stop(id);
+        }
+
+        @Override
+        public boolean clearContext(String id) {
+            clearContextCalls++;
+            return delegate.clearContext(id);
+        }
+
+        int clearContextCalls() {
+            return clearContextCalls;
+        }
+    }
+
     @Test
     void acquireRecordsTheAgentSessionIdFromTheHandle() {
         FakeHerdr herdr = new FakeHerdr();